254 



Dr. A. Arber. On the Development and 



cell-layers develop more rapidly than the inner tissues, and consequently, 

 become wrinkled, or folded in, along certain lines. In relation to the 

 epidermis alone, the word " folded " might perhaps be admissible, but the 

 extension of the term to the leaf as a whole cannot be justified. It may 

 possibly be thought that I am exaggerating the value of a slight distinction, 

 in laying so much stress upon the difference between invagination and 

 folding. But this distinction is by no means so trivial as it may appear at 

 a casual glance, and I believe, moreover, that the impression conveyed by 

 the description of the young Palm leaf as " folded " has been just sufficiently 

 inexact to prevent botanists realising the true morphological nature of the 

 leaves of Palms — a subject to which we shall return in a later section of 

 this paper (p. 260). 



When we come to analyse the process of invagination more closely, we see 

 that it must depend, in the last resort, on a tendency to disproportionate 

 rapidity of growth in the surface layers as compared with those that are more 

 deep-seated. And that such disparity should exist in the leaves is not surprising, 

 when we realise that the Palms have undoubtedly a general tendency towards 

 hypertrophy of the superficial tissues of their various organs. The results of 

 this tendency are witnessed in the non-vascular spines which many Palms 

 develop on the leaf-rachis, and in the squamiform ovarial hairs, which, in such 

 cases as Eaphia Ruffia, Mart., form* eventually a woody coat to the fruit. The 

 comparatively large size of the root-cap in the Palms may also be a case in 

 point, for the calyptrogen, in the Monocotyledonous root, is the only tissue 

 which is strictly epidermal. Possibly the integumental outgrowths, which 

 ruminate the endosperm in many genera, may be regarded as another 

 expression of the same tendency, while a further example is afforded by the 

 proliferations producing the " coiffe " of the young leaf — a structure which 

 we shall consider in the following paragraph. 



2. The Membrane (" Coiffe "). 



It has long been known that the lamellte of the very young leaves of certain 

 Palms are connected by a kind of ephemeral membrane — the " Hiille," " Haut," 

 " coiffe," or " pellicule " — so that the limb resembles a closed fan sheathed in 

 tissue paper. This membrane is ruptured when the leaf unfolds. The view 

 taken as to the nature of this envelope necessarily depends upon that held 

 regarding the origin of the plications. To Von Mohl, Trecul and Kallmann, 

 the membrane was a primary constituent of the leaf, and represented the 

 survival of the superficial tissues which were not involved in the splitting 

 which these authors postulated. By those, on the other hand, who believe 

 that the plications originate by folding, the membrane is held to be a secondary 



